An S-1 filing is a form required by the US markets regulator, the Securities Exchange Commission, when a company is filing to go public and wants to sell its shares to stock market investors. An S-1 filing is a landmark moment for many companies, similar to companies in India filing their Draft Red Herring Prospectus (DRHP) with market regulator SEBI to go public. There is a key difference though. Unlike in India, US laws allow companies to file confidentially for an IPO, and the process can be quicker and smoother. Companies can go from filing their S-1 to actually being traded in weeks.